1-pager: go/tablet-mode-screenshot-hotkey

changes:
- Creates PowerButtonScreenshotController for handling tablet mode
  power button screenshot accelerator. It is android-like accelerator now,
  which requires pressing power button and volume down simultaneously, the
  delay currently is set to 150 ms.
- Makes volume down wait 150 ms (do not propagate it), and if power button
  is pressed, taking a screenshot, and do not propagate any volume down
  events.
- For tablet power button, volume down or volume up key pressed
  cancels/invalidates the tablet power button behavior.
- For forced clamshell power button (eve), make power button wait 150 ms,
  if volume key pressed, do not start the pressed clamshell power button.
- Removes old power button screenshot logic code.
- Add test coverage for new power button screenshot code.

Follow-up work:
- UMA: Record the delay between power button pressed and volume-down pressed.

button, also added test coverage in
power_button_screenshot_controller_unittest.cc

Bug: 712072
Test: device test for both tablet power button and forced clamshell power
Change-Id: I31e0a49317079557904a9e435f210a532364acc9
Reviewed-on: https://chromium-review.googlesource.com/704193


Commit-Queue: Qiang(Joe) Xu <warx@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509479}